United States Patents
______________________________________ 3,937,792 February 10, 1976 R. J. Lumby 4,127,630 November 28, 1978 M. E. Washburn 4,869,943 September 26, 1989 N. D. Corbin et al. ______________________________________
Silicon nitride has recently become a very promising material for structural ceramic applications due to its high strength properties and its excellent electrical insulation properties. Thus, the demand for commercial quantities of silicon nitride has increased dramatically and there are many efforts underway to manufacture silicon nitride powder in a more economical, safer manner and in a purer form.
The most common current process for manufacturing silicon nitride powder entails nitriding a dry, relatively loose-packed bed of silicon powder containing up to about 20 wt % silicon nitride seed. See for example, U.S. Pat. Nos. 3,937,792 and 4,127,630. As defined herein a "bed of silicon powder" comprises silicon powder which has been formed by some means, e.g. slip cast or pressed or otherwise, into a cake. The pore structure of such a loosely-packed silicon bed is very large, broad and non-uniform. As a result, during nitriding whiskers often grow into any voids that are greater than about ten microns. Such whiskers may be a health hazard which makes the nitriding process somewhat dangerous to run.
To prepare silicon nitride powder from the solid silicon nitride bed, the bed must first be subjected to a coarse crushing (e.g. in a hammerhill or jaw crusher) to produce pieces which are of a size suitable for subsequent final milling into silicon nitride powder. The coarse crushing step is quite energy intensive, generates considerable dust, and also potentially contaminates the powder with iron and/or tungsten carbide from the equipment.
It is an object of the present invention to produce a friable silicon nitride body which can be directly final milled, thereby overcoming the problems of the prior art process and safely and economically producing a commercial grade of silicon nitride powder, particularly one that contains little or no silicon nitride whisker contamination.